Zoom into the H-R diagram where C-stars are located. The solar metallicity isochrones (Marigo et al. 2008) are plotted in grey and the small numbers indicate the predicted present mass. The sampling of points in the isochrones is shown with tiny squares. The position of the C-stars is indicated by a shaded area, the uncertainty about this area is discussed in Sect. 5. The different symbols plotted correspond to the parameters determined for every star and distance assumed: plus for CR Gem, asterisk for HK Lyr, diamond for RV Mon, triangle for Z Psc, and square for DR Ser.